The present invention relates to a communication apparatus which receives, in an industrial process for example, a current signal flowing through a 2-wire system transmission line to control such loads as valves, etc. and which transmits a signal indicating a control condition, etc. by changing an in-line voltage of the transmission line.
In industrial processes, a receiving apparatus called a positioner is generally provided for remotely controlling valves, etc. But in the case of such field apparatus, a signal is transmitted from the central control unit by a current value which changes in the range, for example, of 4-20 mA and this signal is received by the receiving apparatus. Thereby, controls are carried out in accordance with a current value.
However, the apparatus of the prior art has the disadvantage that a 2-wire system transmission line is required for transmission of a current value indicating the signal and simultaneously another 2-wire system transmission line is also required in order to supply the required power to the receiving apparatus. Namely, a 4-wire system transmission line is essential. Thus, the required amount of wire material for the transmission line and the man-hours for wiring increase and the facility cost also becomes high.
The prior art also has the disadvantage that an additional transmitting apparatus is required and it must be connected with the control unit by an exclusive transmission line in order to monitor the control and operating conditions of valves, etc. and thereby an uneconomical investment is required.